Improve font handling

[SteelWagstaff]

At present, Pressbooks-book bundles dozens of typeface files that are used by various Pressbooks themes: https://github.com/pressbooks/pressbooks-book/tree/dev/assets/book/typography/fonts. These could be excluded from the book theme itself and requested/downloaded only as needed, as we now do when adding extra script/language support or Thema symbionts.

See https://github.com/fontsource/fontsource and https://github.com/google/fonts#sync-with-git for some possible approaches.